It is becoming increasingly more apparent that refrigerant substitutes must be found to replace chlorofluorocarbons (CFC's) which have been found to be a major contributor to the depletion of the ozone layer. Commercial development has led to advances in the manufacture and use of refrigerants which do not contain CFC's. For example, in many refrigerant applications, the long-standing and widely-used refrigerant Freon or R-12 is being replaced by the non-chlorinated, fluorinated refrigerant HFC-134a (1,1,1,2-tetrafluoroethane). Ammonia has long served as a refrigerant and continues to be an important refrigerant. Ammonia has been found to have no effect on the depletion of the ozone layer and, equally as important, ammonia does not contribute to the greenhouse effect. The greenhouse effect is the gradual warming of the earth's atmosphere due to the build-up within the atmosphere of certain greenhouse gases such as CO.sub.2 and NO.sub.2. Because ammonia has a very brief atmospheric life, it does not contribute to the buildup of greenhouse gasses.
In addition, ammonia has many attractive advantages such as being a highly efficient refrigerant at a relatively low cost. On the down side, the major disadvantages of using ammonia as a refrigerant are due to its toxicity and, to a certain extent, to its flammability. However, these disadvantages have led to improved compressor and system designs which provide for more impervious barriers to prevent the escape of ammonia refrigerant from the system. Also, because of its distinctive and easily detectable odor, ammonia leaks can be more easily detected than certain other refrigerants and quickly eliminated.
The use of ammonia as a refrigerant has been limited to a certain extent due to physical and chemical interactions of ammonia with traditional refrigeration compressor lubricants. These limitations are generally the result of a lack of miscibility (liquid ammonia with lubricant) and solubility (gaseous ammonia with lubricant) of ammonia with conventional lubricants which interferes with the efficient transfer of heat and, in some cases, limits the efficient use of ammonia with certain types of heat exchangers.
It is well known in the art that traditional refrigeration lubricants such as mineral oil and synthetic hydrocarbon fluids/oils become less soluble with ammonia as temperature decreases and, thus, the lubricant can separate or drop out into system low spots such as intercoolers, suction accumulators, and evaporators..sup.1 As the oil migrates to the low spots in the system, it becomes necessary to add more oil to the compressor, thereby further perpetuating the problem. Elaborate means which normally require the lubricant to be drained manually from the system, such as oil stills and drain connections at the bottom of evaporators, recirculators, intercoolers, etc., have been used to remove the oil.
In the evaporator where ammonia is present in liquid form, mineral oils and synthetic hydrocarbon oils are immiscible with the liquid ammonia and the oil tends to "foul" heat exchange surfaces causing a loss of heat transfer efficiency. In evaporators where the ammonia refrigerant is present in gaseous form, mineral oils become viscous due to a lack of solubility and tend to build up in thick film on the heat transfer surfaces. This increased viscosity not only causes a loss of heat transfer efficiency, but restricts the flow of the refrigerant causing increased pressure within the system contributing to further losses in the efficiency of the system.
The function of a compressor lubricant is to provide adequate lubrication to compressor parts. To best perform this function, the lubricant should remain in the compressor rather than circulating through the entire system. Oils having low volatility characteristics will not turn into vapor at compressor discharge temperatures and, thus, may be removed with oil separators. It is inevitable, however, that the oil will naturally come into contact with the refrigerant in the compressor where it is entrained by the refrigerant in the form of small particles. Discharge side oil separators generally are not 100% efficient at separating the oil from the refrigerant, thus a certain amount of oil will pass to the condenser and the liquid receiver where it will be carried by the liquid refrigerant into the evaporator.
The presence of oil circulating through the system adversely effects the efficiency and capacity of the entire system. The major reason for this is the tendency of the oil to adhere to and to form a film on the surfaces of the condenser and evaporator tubes (or surfaces) reducing the heat transfer capacity of the condenser and the evaporator tubes. The effect of an oil film in evaporators has been shown to decrease the efficiency of a system, which can easily be 20% in an air cooler.sup.2 to 40% or more, with increasing oil film thickness, in brine chillers..sup.1 It is obvious that it is desirable to maintain both compressor lubrication and system efficiency. This can best be accomplished by a lubricant with a low volatility which can be easily returned from the system to an oil reservoir where it can perform its intended lubrication function.
The Mobil Oil Corporation publication "Refrigeration Compressor Lubrication with Synthetic Fluids", which is incorporated herein by reference, discusses systems of the type with which the present invention finds use. Evaporators may be classified according to the relative amount of liquid and vapor refrigerant that flows through the evaporator. The so called dry expansion evaporator is fed by means of a flow control device with just enough refrigerant so that essentially all of the refrigerant evaporates before leaving the evaporator. In a flooded evaporator, the heat exchange surfaces are partially or completely wetted by a liquid refrigerant.
A direct expansion (DX) coil is one example of an evaporator in which a liquid refrigerant and a certain amount of flash gas is present as the refrigerant enters the evaporator. Flash gas is gas which appears when a refrigerant as a saturated liquid passes through an expansion valve undergoing a drop in pressure and instantaneously forming some gas, i.e., flash gas. As the refrigerant moves downstream through the system, the proportion of vapor increases until essentially all of the refrigerant is in vapor form before exiting the evaporator.
Shell and tube and flooded coil evaporators are both typical examples of flooded evaporators. In flooded evaporators, all of the heat transfer surfaces are wetted by the liquid refrigerant.
In an ammonia flooded evaporator, conventional mineral oils and synthetic hydrocarbon oils are essentially immiscible with ammonia. Any amount of oil entering the system tends to foul the heat transfer surfaces resulting in a loss of system efficiency. Because the oils typically are heavier than liquid ammonia, provisions must be made to remove the oil from low areas in the evaporator, as well as other low areas in the system. Additionally, an oil separator is almost always required.
In direct expansion evaporators using soluble halocarbon refrigerants, refrigerant velocity must be maintained at a sufficiently high rate at the heat exchanger outlet to effectively return the lubricant to the compressor. One study with R-12 in mineral oil.sub.3 indicates that an oil which is miscible and has an oil content of less than 10% will have little or no effect on the heat transfer coefficient. However, it is desirable to keep oil concentration low due to the effect on pressure caused by the oil. As the oil/refrigerant mixture passes through the heat exchange tubes, it increases in viscosity due to both reduction in temperature and increased oil concentration. The increased oil concentration results in a pressure increase. This suggests that an oil/refrigerant mixture with a lower operational viscosity, particularly with some dissolved refrigerant, will reduce the effect on pressure resistance.
In the case of ammonia, normal naphthenic or paraffinic lubricants and synthetic hydrocarbon fluids/oils have low solubility and miscibility in ammonia. These oils are heavier than ammonia and tend to form an oil film on the heat transfer surfaces, or "foul", decreasing the system capacity and efficiency. The low solubility inherent with these oils also results in less dilution by the ammonia and a greater increase in refrigerant in direct expansion systems. The oil film, then, can become too thick for efficient heat transfer thereby contributing to excessive pressure increases in the evaporator and restricted oil return to the compressor.
Recently, welded plate and hybrid cross-flow plate evaporators have been proposed which would provide significant reductions in required refrigerant volume for ammonia systems. The reduction in required refrigerant volumes allows for the achievement of efficient heat transfer while also reducing the potential for ammonia refrigerant leakage..sup.4 The reduction in refrigerant charge volumes also enables ammonia to be safely permitted for use in a much wider variety of applications in addition to its common industrial applications. Further advantages of this type of system design includes lower system cost and reduced system size and weight. However, in order to take full advantage of this type of evaporator system, it would be desirable to use lubricants which have both a minimum effect on heat transfer efficiency and a minimum of pressure restriction in the evaporator.
Most lubricants used for refrigeration compressors with ammonia as a refrigerant are lubricated with an oil with an ISO viscosity grade (VG) of 32-68, where the ISO VG represents the approximate viscosity of the oil at 40.degree. C. In some cases, such as with some rotary screw compressors, the ISO VG can be as high as 220. Because normal evaporators operate at a temperature of approximately -40.degree. C., it is desirable to have a lubricant that is a fluid at -40.degree. C. In some cases, synthetic oils are used for evaporator temperatures below -40.degree. C., as conventional oils are usually solid at these temperatures. Improving the low temperature fluidity through selection of an oil which has a lower viscosity at evaporator temperatures helps to improve oil return. Improving the low temperature oil return represents a partial solution to the problem of the fouling of heat transfer surfaces.
Generally, with immiscible oils, a reduction in oil concentration results in a reduction in terminal oil film thickness and also increases the amount of time for the oil to reach this thickness..sup.2 Constant removal of oil from the system, which is assisted through improved fluidity, is one method to reduce oil concentration.
Another method useful for reducing oil concentration is to decrease the amount of oil entering the system. Oil separators are designed to remove nearly all of the liquid oil from the compressor discharge vapor. Unfortunately, these separators cannot remove oil which is in vapor form. Oil vapor passes through these separators and condenses in the condenser together with the ammonia vapor and eventually flows to the evaporator. The efficiency of these oil separators is such that the oil concentration can be as little as 0.2 parts per million in mass in the ammonia refrigerant at saturation temperatures of 25.degree. C. to over 70 parts per million in mass at 100.degree. C. when conventional oils are used.
The miscibility of mineral oils and synthetic hydrocarbon oils in ammonia is generally limited to less than one part per million in mass..sup.2 Oil scrubbers have been proposed to eliminate oil from entering the system..sup.2 Oil scrubbers may be suitable for large systems but are often considered undesirable for smaller systems, especially those with direct expansion evaporators where it is desirable to reduce the amount of ammonia in the system and limit weight through elimination of unnecessary piping and accessories.
Attempts have been made to overcome the problems associated with the use of ammonia refrigerant with direct expansion evaporators. An example of this is German patent DE 4202913 A1 which discloses the use of conventional mineral oil circulating through so-called dry evaporator (direct expansion). However, the circulation through the dry evaporator is limited due to both poor solubility of the ammonia refrigerant in the mineral oil lubricant and due to poor low temperature viscosity of the mineral oil lubricant. The resulting restriction to the evaporation of ammonia caused by the oil prevents efficient heat transfer.
The use of dry evaporators (direct expansion) with ammonia refrigerant is desirable, particularly in installations of relatively small and medium sized capacity, as the refrigerant capacity and, therefore, the hazard of escaping ammonia is reduced. The German patent DE 4202913 A1 also teaches the use of low molecular weight amines such as mono-, di-, and trimethylamine which are added to the ammonia refrigerant to enhance the solubility of the conventional oil (mineral oil) in the ammonia refrigerant. However, the use of amines can result in additional problems with safety. The flash point for these amines ranges from -10.degree. C. or monomethylamine to -12.2.degree. C. or trimethylamine. A further safety issue involves the explosive limits in air for these two amines. Monomethylamine has an explosive limit in air of 5-21%; trimethylamine has an explosive limit in air of 2-11.6%. Both of these amines are classified as being dangerous fire risks. Although ammonia is known to be flammable, the range of flammability is limited to concentrations in the air of between 16-35%. The addition of the amine component to increase the solubility of the ammonia refrigerant in the conventional mineral oil lubricant amplifies the hazardous nature of the combination and thereby limit its possible applications.
Japanese Patent Application No. 5-9483 to Kaimai et al. discloses a lubricant for ammonia refrigerants which is a capped polyether compound containing organic oxides. The Kaimai et al. reference uses R groups (R, R.sub.1 -R.sub.10) which are alkyl groups having less than ten carbons in length, preferably are less than four carbons in length, to cap the ends of the lubricant molecule. Kaimai et al. teaches that the total number of carbons (exclusive of the organic oxide groups) suitable for polyether lubricants is 8 or below with alkyl groups of 1-4 carbons being preferred. Polyether lubricant compounds of greater than eight carbons were discouraged by Kaimai et al. due to incompatibility with ammonia.
Matlock and Clinton in the chapter entitled "Polyalkylene Glycols" in Synthetic Lubricants and High Performance Functional Fluids, which is incorporated herein by reference, discusses the class of synthetic lubricants called polyalkylene glycols. Polyalkylene glycols, also known as polyglycols, are one of the major classes of synthetic lubricants and have found a variety of specialty applications as lubricants, particularly in applications where petroleum lubricants fail. Because ammonia is more soluble in polyglycols than synthetic hydrocarbon fluids or mineral oils, it was thought that polyglycols would not offer any efficiency benefits in ammonia refrigeration systems..sup.6
Polyalkylene glycol is the common name for the homopolymers of ethylene oxide, propylene oxide, or the copolymers of ethylene oxide and propylene oxide. Polyalkylene glycols have long been known as being soluble with ammonia and have been marketed for use in ammonia refrigeration applications.
U.S. Pat. No. 4,851,144 to McGraw et al., teaches a lubricant composition including a mixture of a polyalkylene glycol and esters. McGraw discloses conventional polyglycol lubricants for hydrofluorocarbon refrigerants having a hydrocarbon chain of C.sub.1 to C.sub.8. In order to increase the miscibility of the lubricants, McGraw teaches the addition of esters. The use of esters with ammonia lubricants is contraindicated due to the immediate formation of sludges and solids which foul heat transfer surfaces and reduce overall system efficiency.
Because polyalkylene glycols are polar in nature and, therefore, water soluble, they are not very soluble in non-polar media such as hydrocarbon. The insolubility of polyalkylene glycols in non-polar media make them excellent compressor lubricants for non-polar gasses such as ethylene, natural gas, land fill gas, helium, or nitrogen (Matlock and Clinton at page 119). Because of this polar nature, polyalkylene glycols have the potential for further becoming highly suitable lubricants for use with ammonia refrigerants. However, the same polar nature which allows polyalkylene glycols to be soluble in ammonia is the same property which allows polyalkylene glycols to be soluble in water. Solubility with water has been a long-standing concern in ammonia refrigeration applications. The presence of excessive water can result in corrosion of the refrigeration system. Bulletin No. 108 of the International Institute of Ammonia Refrigeration entitled, "Water Contamination in Ammonia Refrigeration Systems", .sup.7 which is incorporated herein by reference, discusses the prevailing concerns associated with water contamination of ammonia refrigeration systems. The high specific volume of water as a vapor results in the need for large equipment or, conversely, if water is allowed to accumulate in excessive amounts, equipment designed for ammonia refrigeration would eventually become undersized due to the displacement of the refrigerant by the excess water volume.
It is not uncommon, especially in larger ammonia refrigeration systems, for moisture to enter the system. In the case of ammonia refrigeration systems using mineral oil lubricants, water can be easily separated from the oil before it is returned from the system to the compressor. The elimination of water in this case may be accomplished by manually "blowing out" or releasing the water just prior to its entry into the evaporator. However, because the solubility of water in conventional polyalkylene glycols ranges from a few percent to complete solubility, removal of the water becomes a more difficult task.
Another drawback for the use of conventional types of polyalkylene glycols, particularly those containing ethoxylates, as lubricants with ammonia refrigerants is that they may be too miscible to be used with flooded evaporators which were designed for mineral oils. This type of evaporator uses the lack of miscibility of mineral oil with ammonia to effect removal of mineral oil from the evaporator and subsequently returns the oil to the compressor. Because of its higher specific gravity, the mineral oil can then be drained off from the bottom of the system and returned to the compressor.
Very high levels of miscibility and solubility with ammonia can also result in a loss of lubricity. In the case of hydrodynamic lubrication, the viscosity of the oil/refrigerant mixture is important at the operating conditions, i.e., temperature and pressure of the compressor. It may be necessary to use a higher viscosity grade of polyalkylene glycol to provide the desired operating viscosity under diluted conditions for adequate fluid flow. In the case of dry exchange evaporators, the use of a lubricant with an excessively high viscosity may result in excessive diluted viscosity in the evaporator causing the accumulation of the lubricant and thus a restricted flow. This restricted flow can reduce the heat exchange efficiency of the system. Though this situation is somewhat compensated for by the high viscosity index characteristics of the polyalkylene glycols and the near complete miscibility and high solubility in the accompanying dilution of the refrigerant, boundary lubrication in the compressor may suffer because of these highly miscible polyalkylene glycols.
It is well known in the art that mineral oils have a tendency to age in ammonia refrigeration systems. This aging results in the oil breaking down and forming lighter fractions as well as forming a sludge-like material which collects within the system and which is difficult to remove. The lighter fractions contribute to the problems associated with providing an effective method for separating the oil from the refrigerant because the lighter fractions of oil become vapor thereby preventing the oil from entering into the refrigeration system.
The sludge-like materials, which are essentially insoluble in mineral oils, drop out of solution and form deposits which contribute to the "fouling" of heat exchanging surfaces throughout the system and may further interfere with the operation of values and other mechanical devices. It, therefore, becomes imperative to provide a mechanism which prevents the build up of sludge-like materials. One such method would be to provide a lubricant which resists aging..sup.8 Another method would be to provide a mechanism for removing the sludge build-up. The simplest method would be to add fresh oil to the system to flush out or dissolve the sludge-like material. However, mineral oils and synthetic oils have little or no capacity to dissolve the sludge-like materials formed in ammonia refrigeration system.
Because of the good solvency characteristics of polyalkylene glycols, these lubricants could provide a very viable alternative lubricant source for the conversion or retro-fitting of systems previously using lubricants such as mineral oil. That is, by switching to polyalkylene glycol lubricants, the build-up of sludge-like materials can be removed on changeover..sup.5
Heretofore, the prior art in the field of polyalkylene glycol-based lubricants was void of any lubricant which encompassed the necessary properties of refrigeration compressor lubricants for ammonia refrigerants. These key properties include miscibility, solubility, compatibility with mineral oils and synthetic hydrocarbon oils/fluids, low volatility, water insolubility, lubricity, and rheology (viscosity temperature characteristics).
The present invention relates to improved lubricant fluids and their method of manufacture resulting in fluids having an excellent balance of miscibility, solubility, and viscosity, thereby making the fluids excellent lubricants for ammonia compression refrigeration systems. The present invention provides polyalkylene glycol lubricants having better miscibility and solubility characteristics than mineral oils, synthetic hydrocarbon fluids/oils, and previously known polyalkylene glycol lubricants.